Nuclear reactors are used in central-station electric power generation, research and propulsion. A reactor pressure vessel contains the reactor coolant, i.e. water, which removes heat from the nuclear core. Respective piping circuits carry the heated water or steam to the steam generators or turbines and carry circulated water or feedwater back to the vessel. Operating pressures and temperatures for the reactor pressure vessel are about 7 MPa and 288.degree. C. for a boiling water reactor, and about 15 MPa and 320.degree. C. for a pressurized water reactor. The materials used in both boiling water and pressurized water reactors must withstand various loading, environmental and radiation conditions.
Some of the materials exposed to high-temperature water include carbon steel, alloy steel, stainless steel, nickel-based alloys, and cobalt-based alloys. Despite the careful selection and treatment of these materials for use in water reactors, corrosion occurs on the materials exposed to the high-temperature water. Such corrosion contributes to a variety of problems, e.g., stress corrosion cracking, sticking of pressure relief valves, buildup of the gamma radiation emitting .sup.60 Co isotope and erosion corrosion.
Stress corrosion cracking is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high-temperature water. As used herein, the term "stress corrosion cracking" (hereinafter "SCC") means cracking propagated by static or dynamic stressing in combination with corrosion at the crack tip. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stress from welding, cold working and other asymmetric metal treatments. In addition, water chemistry, welding, heat treatment, and radiation can increase the susceptibility of metal in a component to SCC.
It is well known that SCC occurs at higher rates when oxygen is present in the reactor water in concentrations of about 5 parts per billion (ppb) or greater. Stress corrosion cracking is further increased in a high radiation flux where oxidizing species, such as oxygen, hydrogen peroxide, and short-lived radicals are produced from radiolytic decomposition of the reactor water. Such oxidizing species increase the electrochemical corrosion potential of metals. Electrochemical corrosion is caused by a flow of electrons from anodic and cathodic areas on metallic surfaces. The corrosion potential is a measure of the thermodynamic tendency for corrosion phenomena to occur, and is a fundamental parameter in determining rates of, e.g., SCC, corrosion fatigue, corrosion film thickening, and general corrosion.
Stress corrosion cracking in boiling water nuclear reactors and the associated water circulation piping has historically been reduced by injecting hydrogen in the water circulated therein. The injected hydrogen reduces oxidizing species in the water, such as dissolved oxygen, and as a result lowers the corrosion potential of metals in the water. However, factors such as variations in water flow rates and the time or intensity of exposure to neutron or gamma radiation result in the production of oxidizing species at different levels in different reactors. Thus, varying amounts of hydrogen have been required to reduce the level of oxidizing species sufficiently to maintain the corrosion potential below a critical potential required for protection from SCC in high-temperature water. As used herein, the term "critical potential" means a corrosion potential at or below a range of values of about -230 to -300 mV based on the standard hydrogen electrode (she) scale for the case of pure water. Stress corrosion cracking proceeds at an accelerated rate in systems in which the electrochemical potential is above the critical potential, and at a substantially lower rate in systems in which the electrochemical potential is below the critical potential. Water containing oxidizing species such as oxygen increases the corrosion potential of metals exposed to the water above the critical potential, whereas water with little or no oxidizing species present results in corrosion potentials below the critical potential.
In a boiling water reactor (BWR), the radiolysis of the primary water coolant in the reactor core causes the net decomposition of a small fraction of the water to the chemical products H.sub.2, H.sub.2 O.sub.2 and O.sub.2. For steady-state operating conditions, equilibrium concentrations of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 are established in both the water which is recirculated and the steam going to the turbine. This concentration of O.sub.2, H.sub.2 O.sub.2, and H.sub.2 is oxidizing and results in conditions that can promote SCC of susceptible materials of construction. One method employed to mitigate SCC of susceptible material is called hydrogen water chemistry, whereby the oxidizing nature of the BWR environment is modified to a more reducing condition. This effect is achieved by adding hydrogen gas to the reactor feedwater. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species to reform water, thereby lowering the concentration of dissolved oxidizing species in the water. The rate of these recombination reactions is dependent on local radiation fields, flow rates and other variables.
Corrosion potentials of stainless steels in contact with reactor water containing oxidizing species can be reduced below the critical potential by injection of hydrogen into the water in a concentration of about 50 to 100 ppb or greater. For adequate feedwater hydrogen addition rates, the conditions necessary to inhibit SCC can be established in certain locations of the reactor. These conditions are an electrochemical potential of less than -0.230 V.sub.she. Different locations in the reaction system require different levels of hydrogen addition, as shown in FIG. 2. Much higher hydrogen injection levels are necessary to reduce the corrosion potential within the high radiation flux of the reactor core, or when oxidizing cationic impurities, e.g., cupric ion, are present.
However, feedwater hydrogen additions, e.g., of about 200 ppb or greater, that reduce the corrosion potential below the critical potential, can result in a higher radiation level in the steam-driven turbine section from incorporation of the short-lived .sup.16 N species, as shown in FIG. 3. For most BWRs, the amount of hydrogen addition required to provide mitigation of SCC of pressure vessel internal components results in an increase in the main steam line radiation monitor ("MSLRM") by a factor of greater than about four. This increase in main steam line radiation can cause high, even unacceptable, environmental dose rates that can require expensive investments in shielding and radiation exposure control.
Accordingly, although the addition of hydrogen lowers the corrosion potential of reactor water, it is also desirable to limit the amount of hydrogen in reactor water, while maintaining the corrosion potential below the critical potential.
The primary products of water radiolysis in the core are H.sub.2, H.sub.2 O.sub.2, OH, H and the hydrated electron. In irradiated water, O.sub.2 and H.sub.2 O.sub.2 are in a state of dynamic equilibrium. During HWC, the computed ratio of H.sub.2 O.sub.2 to O.sub.2 in the downcomer annulus is large. The reason reported by M. Ullberg et al., "Hydrogen Peroxide in BWRs", Water Chemistry for Nuclear Reactor Systems 4, BNES, London, 1987, pp. 67-73, is that the H.sub.2 added during HWC initially slows down the oxidation of H.sub.2 O.sub.2 to O.sub.2, speeds up the reduction of O.sub.2 to H.sub.2 O.sub.2 and has little effect on the reduction of H.sub.2 O.sub.2 to H.sub.2 O. Thus, hydrogen peroxide is relatively stable in the recirculation water of a BWR.
It is further known from the Ullberg et al. article that H.sub.2 O.sub.2 in water will decompose on a heterogeneous solid surface at elevated temperatures by the reaction: EQU 2H.sub.2 O.sub.2 +Surface.fwdarw.2H.sub.2 O+O.sub.2
This decomposition of H.sub.2 O.sub.2 is referred to as heterogeneous decomposition. The rate of decomposition can be increased through the use of decomposition catalysts and will also be dependent on the temperature and the ratio of surface area to volume.